Liquid treating apparatus for recovering pure water from polluted water using a mixture of different ion exchange resins is being widely used. In such liquid treating apparatus, as the treatment proceeds some of the cation exchange resin particles react with cations and some of the anion exchange resin particles react with anions to remove the ions from the water. However, there is a critical point where the ion exchange resin particles will be nearly saturated with cations or anions in the water after a certain period of reaction thus causing a reduction of treatment capacity and leaving most ions untreated. Such nearly ion-saturated ion exchange resins are subjected to a recovering process. Ion exchange resin saturated with different ions must be treated with different recovering agents. For example, a cation exchange resin nearly saturated with cations in the water is treated with sulfuric acid or the like while an anion exchange resin nearly saturated with anions in the water is treated with caustic soda or the like.
To prepare for the recovering process, the two kinds of ion exchange resins are separated from each other by utilizing their difference in specific gravity and their corresponding different sedimentation rates.
In a resin recovering column for the above separation process, the process must be monitored to see that normal separation is carried out. In the past, such monitoring has been done by observing a change in color. Generally, a cation exchange resin and an anion exchange resin differ in color from each other, for example dark brown for cation exchange resins and yellow brown for anion exchange resin. Such color difference has been checked visually.
More specifically, the side wall of the resin recovering column is provided with a viewing window extending longitudinally. The observer looks through the viewing window to check the difference in color of the layer structures of the anion exchange resin, which tends to form an upper layer because of its lower sedimentation rate caused by its lower specific gravity in a back washing separation process, and the cation exchange resin, which tends to form a lower layer because of its higher sedimentation rate caused by its higher specific gravity in the same back washing separation process, and the level of the boundary therebetween. From such a visual check the observer will determine how far the separation process has gone and whether proportions of the cation ion exchange resin to anion exchange resin are normal or whether the proper amount of the resin is being supplied.
An example of such prior art is provided by the patent to Burgess U.S. Pat. No. 3,429,807 and the patent to Stanley Jr. U.S. Pat. No. 3,634,229 which disclose a monitoring apparatus using a viewing glass or transparent tank through which the observer can easily check the level of the boundary between the cation exchange resin and the anion-exchange resin.
However, a visual check of the ion exchanging resin separation process is disadvantageous in that the resin separation operation must be interrupted and the observer must travel between the control panel and the recovering column to effect the visual check, thus making full automation of the recovering process impossible. It is troublesome for operators to travel between the control panel and the recovering column. Moreover, the prior art monitoring systems are not desirable from the viewpoint of safety and hygenic requirements when employed in atomic energy facilities, since operators may be harmed by irradiation from radioactive resins in the recovering column.
An attempt to automate the monitoring systems is exemplified by U.S. Pat. No. 4,120,786 to Petersen (corresponding to Japanese Patent Public Disclosure No. 56078/79) which discloses an apparatus having an elongated float responsive to the change in the density at the boundary between a cation exchange resin and an anion exchange resin. The float is suspended in a treating column charged with said two kinds of resin, and typically comprise a Westphal displacement element the top end of which is connected to a load cell. In operation, the load cell converts the displacement of the float to an electric signal which will be properly read. Thus, remote monitoring of the level of the boundary between the two kinds of resin is made possible. However, such an apparatus is not capable of continuous monitoring of the step of separation during the process while enabling a remote monitoring of the boundary level of two kinds of resin which have completely been separated. Moreover, such apparatus is not capable of determining automatically when the separation is completed. Thus, this apparatus was not suitable for fully automated separation process.
An example of apparatus for measuring the boundary level between two kinds of materials, typically a cation exchange resin and an anion exchange resin, charged in a treating column, by utilizing the color difference between the two kinds of resin is provided by Japanese Utility Model Public Disclosure No. 9533/81 (Application No. 178299/79) which discloses an apparatus having a plurality of color sensors comprising a light-projecting section and a light-receiving section each sensor of which is secured to a viewing window in alignment with each position to be sensed. In operation, respective color sensors project rays of light on the resin at the corresponding positions in the treating column. The resin reflects the lights and the color sensors receive the reflected lights. In the color sensors photoelectric transducers convert the respective reflected lights to a plurality of electric signals each corresponding to the spectrum received. The electric signals thus obtained are displayed. Thus, remote-monitoring is made possible.
However, such an apparatus is disadvantageous in that although it provides remote monitoring of the boundary level between two kinds of resin which have completely separated, a plurality of (generally a large number of) color sensors must be the same in characteristics. This makes adjustment and maintenance of the system difficult. Further, completion of separation cannot be automatically determined. Thus, this apparatus is not suitable for a fully automated separation process.